To provide an anti-coming-off implement for a sliding door, which has a simple structure.

This anti-coming-off implement 6 for the sliding door, which is mounted on the upper section of the sliding door traveling on a sill and guided by a guide rail 7 of a head 2, comprises a protruding member 11 which is provided in such a manner as to be protrusible upward from the upper section of the sliding door, and an adjusting member 12 which adjusts the height of the protrusion of the protruding member 11 with a pinion portion 17 meshed with a rack portion 14 formed on the side surface of the protruding member 11. An operating section 19, which is provided on the one-end side of the adjusting member 12, is operated to rotate the pinion portion 17, so that the height of the protrusion of the protruding member 11 can be adjusted. The sliding door or a fixing member is provided with a fixing portion 30 for rotatably fixing so as not to come off the other-end side of the adjusting member 12 in an anti-coming-off state.